FIG. 1 illustrates a diagrammatic side view of a conventional tone arm structure. 1 is a balance weight. 2 is an arm shaft holder. 3 is the arm shaft. 4 is a head shell. Pick up for reproducing the sound on the record is accomplished by a pick up cartridge 5 which is fixed to head shell 4. In order to apply suitable needle pressure to cartridge 5, the pressure is adjusted by moving the balance weight along the arm of the freely moving tone arm structure. The fulcrum 0 is in the center of rotation of the arm shaft holder 2. The weight of balance weight 1 must have a gravitational moment which can be in equilibrium with the gravitational moment of the arm shaft 3, the head shell 4 and the cartridge 5 centering on fulcrum 0. In order to reduce the inertial mass of such a tone arm structure, the balance weight 1 should be somewhat heavier and should be positioned somewhat nearer arm shaft holder 2.
FIG. 2 is a side cross-section of the conventional method of affixing balance weight 1. The arm shaft 3 passes tightly through a hole or screw hole in balance weight 1. The balance weight 1 is linked to the tone arm structure and is supported by it. Adjustment of needle pressure of the cartridge is carried out by slight movement along arm shaft 3 of balance weight 1. The position of balance weight 1 after adjustment of the needle pressure is as shown in FIG. 2. In order to find the inertial moment I of balance weight 1 at that time, the following formula is used in which W is the weight of balance weight 1; G is the center of gravity of balance weight 1 and L is the distance from the center of gravity to the fulcrum 0 of the tone arm. ##EQU1## The inertial moment denoted by A is the inertial moment value when a body of weight W moves at rotational radius L centered on fulcrum 0. The inertial moment denoted by B is the value when the body moves with the center of gravity G as the center of rotation. FIG. 2 illustrates a configuration of balance weight 1. The I.sub.G of term B in formula (1) is expressed as follows for such a cylindrical shaft whose radius is a and length is l. EQU I.sub.G =(1/12) Wl.sup.2 ( 2)
Reference should also be made to Japanese Pat. No. 51-23702 entitled "Moving Control Device of Tonearm" for illustration of prior art.